1. Field of the Invention
The present invention relates to an optical wavelength multiplexing and demultiplexing device used in optical communications and a photodetector using the same.
2. Related Background Art
A directional coupler having two asymmetrical waveguides and a grating, as described in Japanese Laid-Open Patent Application No. 2-239209 is proposed as a conventional optical wavelength multiplexing and demultiplexing device for multiplexing or demultiplexing an optical signal. Advantageous features of such an element are a pass band of about several nm, transmission of an optical signal having a nonfiltering wavelength, and the like. An odd mode .epsilon..sub.odd and an even mode .epsilon..sub.even having an electric field distribution shown in FIG. 1 are present in the directional coupler. The grating is used as a phase matching means for coupling these two modes.
As shown in FIG. 1, input signal light consists of wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.e, . . . , .lambda..sub.n. When this input signal light is incident on a first waveguide 1 serving as an input waveguide, it is coupled to the odd mode .epsilon..sub.odd and propagates. However, the TE-like component of the optical signal having the wavelength .lambda..sub.e satisfying the following equation with respect to a grating pitch .LAMBDA.: EQU .beta..sub.oTE (.lambda..sub.e)-.beta..sub.eTE (.lambda..sub.e)=2.pi./.LAMBDA. (1)
is coupled to the even mode .epsilon..sub.even, and the optical power shifts to a second waveguide 2 serving as a detection waveguide. As a result, wavelength demultiplexing is achieved to detect output signal light which is then transferred to another functional element.
In the above equation, .beta..sub.oTE (.lambda..sub.e) and .beta..sub.eTE (.lambda..sub.e) are propagation constants of the odd and even modes of the TE-like polarized components of the light wave having the wavelength .lambda..sub.e.
The remaining wavelength and polarized components directly pass through the input waveguide and are transferred to the next stage.
When TM-like mode light having the wavelength .lambda..sub.m satisfying the following equation: EQU .beta..sub.oTM (.lambda..sub.m)-.beta..sub.eTM (.lambda..sub.m)=2.pi./.LAMBDA. (2)
is mixed in, this light is demultiplexed in the same manner as described above. When an optical output is to be detected immediately after demultiplexing, the S/N ratio may deteriorate. It is, however, easy to set a larger difference .vertline..lambda..sub.m -.lambda..sub.e .vertline. between these two wavelengths than the multiplexed wavelength range .DELTA..lambda., i.e., EQU .vertline..lambda..sub.m -.lambda..sub.e .vertline.&gt;.DELTA..lambda.(3)
thus posing no practical problem.
The TE- and TM-like modes (polarization) represent modes in which the electric field vectors point in directions parallel and perpendicular to the surface of the substrate, respectively.
According to the conventional example, however, only a specific polarized component having a specific wavelength can be demultiplexed. For this reason, when design is made to extract only TE-like wavelength components of the signal light, all the TM-like polarized light components are transferred to the next stage. That is, in wavelength multiplex communication, this design leads to a signal intensity loss.
An optical signal used in communication is generally transmitted through an optical fiber. A good polarized state of a signal input through a fiber cannot be guaranteed unless a polarization preservation fiber is used. Without the polarization preservation fiber, most of the polarized light components become random. In the worst case, all the optical signal components input to the wavelength demultiplexer become TM mode light components. As a result, a desired signal cannot be demultiplexed at all.
Even if the polarization preservation fiber is used, the phase of an electric field is shifted from that of a magnetic field over a long distance. The polarization plane is rotated, or incident light becomes elliptically polarized light.
When polarization is to be preserved, bending of a fiber at a small radius of rotation is not allowed in consideration of a phase shift in light waves caused by a stress. It is, however, almost impossible to perfectly preserve polarization in consideration of the fiber layout used in an optical LAN.
Both the polarization modes may be extracted by connecting a plurality of conventional elements described above along the light wave propagation direction. According to this method, however, the element length undesirably increases, and the transmission loss also increases.
This also applies to a case wherein multiwavelength light components are simultaneously multiplexed or demultiplexed.
In consideration of only polarization, when a medium has proper nonlinearity, it can cause a polarization mode shift from TE to TM or TM to TE. For this reason, there can be provided a polarization-free filter as described in R. C. Alferness et al., Applied Physics Letters, 39, P. 131 (1981). LiNbO.sub.3 used in this filter is a material of this type having a high nonlinearity. Even with this material, a high coupling efficiency cannot be obtained, and the coupling length or the element length undesirably increases. As compared with a semiconductor material, this material is unsuitable for integration of an optoelectric integrated circuit (OEIC). A material of this type having a high nonlinearity is rarely found in semiconductor materials. Thus, there is not found any semiconductor material having a high nonlinearity and electrical characteristics suitable for an OEIC. As a method of extracting light components having various wavelengths and modes, there may be provided a method of parallelly arranging waveguides and arranging regions for coupling target light components between the waveguides from the light incident side, thereby demultiplexing each target light component in one of the regions. However, this method neither shorten the element length nor eliminate the propagation loss.